1. Field of the Invention
The present invention relates to a magnetoresistive element, and to a thin-film magnetic head, a head assembly and a magnetic disk drive each including the magnetoresistive element.
2. Description of the Related Art
Performance improvements in thin-film magnetic heads have been sought as areal recording density of magnetic disk drives has increased. A widely used type of thin-film magnetic head is a composite thin-film magnetic head that has a structure in which a write head and a read head are stacked on a substrate, the write head incorporating an induction-type electromagnetic transducer for writing, the read head incorporating a magnetoresistive element (hereinafter, also referred to as MR element) for reading.
Examples of the MR element include a GMR (giant magnetoresistive) element utilizing a giant magnetoresistive effect, and a TMR (tunneling magnetoresistive) element utilizing a tunneling magnetoresistive effect.
Read heads are required to have characteristics of high sensitivity and high output. As the read heads that satisfy such requirements, those incorporating spin-valve GMR elements or TMR elements have been mass-produced.
A spin-valve GMR element and a TMR element each typically include a free layer, a pinned layer, a spacer layer disposed between the free layer and the pinned layer, and an antiferromagnetic layer disposed on a side of the pinned layer farther from the spacer layer. The free layer is a ferromagnetic layer having a magnetization that changes its direction in response to a signal magnetic field. The pinned layer is a ferromagnetic layer having a magnetization in a fixed direction. The antiferromagnetic layer is a layer that fixes the direction of the magnetization of the pinned layer by means of exchange coupling with the pinned layer. The spacer layer is a nonmagnetic conductive layer in a spin-valve GMR element, and is a tunnel barrier layer in a TMR element.
Examples of a read head incorporating a GMR element include one having a CIP (current-in-plane) structure in which a current used for detecting a signal magnetic field (hereinafter referred to as a sense current) is fed in the direction parallel to the planes of the layers constituting the GMR element, and one having a CPP (current-perpendicular-to-plane) structure in which the sense current is fed in a direction intersecting the planes of the layers constituting the GMR element, such as the direction perpendicular to the planes of the layers constituting the GMR element.
Read heads each incorporate a pair of shields sandwiching the MR element. The distance between the two shields is called a read gap length. Recently, with an increase in recording density, there have been increasing demands for a reduction in track width and a reduction in read gap length in read heads.
As an MR element capable of reducing the read gap length, there has been proposed an MR element including two ferromagnetic layers each functioning as a free layer, and a spacer layer disposed between the two ferromagnetic layers (such an MR element is hereinafter referred to as an MR element of the three-layer structure), as disclosed in U.S. Pat. No. 7,035,062 B1, for example. In the MR element of the three-layer structure, the two ferromagnetic layers have magnetizations that are in directions antiparallel to each other and parallel to the track width direction when no external magnetic field is applied to those ferromagnetic layers, and that change their directions in response to an external magnetic field.
In a read head incorporating an MR element of the three-layer structure, a bias magnetic field is applied to the two ferromagnetic layers. The bias magnetic field changes the directions of the magnetizations of the two ferromagnetic layers so that each of the directions forms an angle of approximately 45 degrees with respect to the track width direction. As a result, the relative angle between the directions of the magnetizations of the two ferromagnetic layers becomes approximately 90 degrees. When a signal magnetic field sent from the recording medium is applied to the read head, the relative angle between the directions of the magnetizations of the two ferromagnetic layers changes, and as a result, the resistance of the MR element changes. For this read head, it is possible to detect the signal magnetic field by detecting the resistance of the MR element. The read head incorporating an MR element of the three-layer structure allows a much greater reduction in read gap length, compared with a read head incorporating a conventional GMR element.
For an MR element of the three-layer structure, one of methods for directing the magnetizations of the two ferromagnetic layers antiparallel to each other when no external magnetic field is applied thereto is to antiferromagnetically couple the two ferromagnetic layers to each other by the RKKY interaction through the spacer layer.
Disadvantageously, however, this method imposes limitation on the material and thickness of the spacer layer to allow antiferromagnetic coupling between the two ferromagnetic layers. In addition, since this method limits the material of the spacer layer to a nonmagnetic conductive material, it is not applicable to a TMR element that is expected to have a high output, or a GMR element of a current-confined-path type CPP structure, which is an MR element also expected to have a high output and having a spacer layer that includes a portion allowing the passage of currents and a portion intercepting the passage of currents. The above-described method further has a disadvantage that, even if it could be possible to direct the magnetizations of the two ferromagnetic layers antiparallel to each other, it is difficult to direct those magnetizations parallel to the track width direction with reliability.
U.S. Pat. No. 6,169,647 B1 discloses a method of weakly fixing the directions of the magnetizations of the two ferromagnetic layers of an MR element of the three-layer structure so that the magnetizations of the two ferromagnetic layers are directed antiparallel to each other, through the use of two antiferromagnetic layers disposed on the respective sides of the two ferromagnetic layers farther from the spacer layer.
However, this method has a disadvantage that a reduction in read gap length is difficult due to the presence of the two antiferromagnetic layers. In addition, while this method requires that the exchange coupling magnetic fields generated from the two antiferromagnetic layers be directed antiparallel to each other, it is very difficult to subject the two antiferromagnetic layers to such a heat treatment (annealing) that this requirement can be satisfied.